In recent years, as research on genes and proteins has been extensively made, speedy and precision detection of bio-molecules such as protein, bacteria, virus, germ and DNA is required. Detection of bio-molecules employs mainly an antigen-antibody reaction method. Markers for particular pathogens such as cancer markers or cardiac markers are already commercialized in the form of a kit and extensively used in clinical and pathological examinations and the like.
Among the above techniques, the enzyme-linked immunosorbent assay (ELISA) method is explained hereafter. FIGS. 1a to 1c are diagrams illustrating a conventional enzyme-linked immunosorbent assay method. As shown in FIG. 1, in order to detect a particular antibody 12, a conjugated antigen 13 is disposed on a chip 10. The antibody 12 is combined to the conjugated antigen 13 arranged on the chip 10 by means of an antigen-antibody reaction. A fluorescent or isotopic marker 15 is combined to this antibody 12, which is then cleansed. Then, as illustrated in FIG. 1c, the antibody 12 after cleansing is combined with the antigen 13 and simultaneously with the marker 15. Thereafter, the intensity of fluorescence generated by the maker 15 is measured to detect the antibody 12 to be measured. In this enzyme-linked immunosorbent assay method, a combination procedure of the maker 14 is to be added, along with the cleansing and antigen-antibody reaction procedures, thereby resulting in a complicated process. In addition, the fluorescent material of the marker 15 is absorbed in the antibody 12, disadvantageously leading to a reduction in the measuring accuracy.
As a simplified and precision technique, relative to the enzyme-linked immunosorbent assay method, a susceptibility attenuation immuno-assay method has been developed. FIGS. 2a and 2b are schematic diagrams illustrating a convention susceptibility attenuation immuno-assay method using magnetic nanoparticles. As shown in FIG. 2a, this method employs a magnetic agent where an antigen 26 is attached to a magnetic nanoparticle 24 having a size of a bio-molecule. Here, the antigen 26 is a conjugated antigen which has a good affinity with the antibody 27 to be measured. Each of the respective magnetic nanoparticles 24 has a self magnetic moment and is enclosed with a layer 25 not to be agglomerated with each other. For example, the layer 25 is formed of Dextran.
As illustrated in FIG. 2b, if an external AC magnetic field is applied to the magnetic nanoparticles 24, the magnetic nanoparticles 24 spin in resonance with the external AC magnetic field. Here, the nanoparticles having different sizes spin in response to the AC magnetic fields having different frequencies. The magnetic nanoparticles, which spin in resonance, generate a magnetic field having the same frequency as the external AC magnetic field, thereby exhibiting a high susceptibility. If the magnetic nanoparticles 24 have a uniform size, the magnitude of oscillation in the magnetic moment exhibits a maximum peak at a particular frequency. Here, if an antibody 27 to be measured is contained in the magnetic agent solution, the magnetic nanoparticles 24, to which the antigen 26 is attached, is combined with each other by the antigen 27. The combined magnetic nanoparticles 30 cannot easily spin in response to a change in the external magnetic field, and consequently the antibody 27 in the magnetic agent reduces the AC susceptibility of the agent containing the magnetic nanoparticle 24. If the susceptibility where the antibody 27 is contained in the magnetic agent is normalized as being susceptibility when in absence of no antibody, a calibration curve can be obtained regardless of concentration of the magnetic agent and also a quantitative concentration of the antibody 27 can be determined.
That is, an AC magnetic field having a particular frequency and the change in the magnitude of magnetic field, which is generated by the magnetic nanoparticles 24 contained in the magnetic agent, is measured with precision, thereby enabling to detect the antibody 27 contained in the magnetic agent.
In this case, the detection device employs in generally an inductive detector using a coil. However, the inductive detector has a weak sensitivity to disadvantageously degrade the measuring sensitivity thereof. Thus, as a high sensitivity detector, a high-temperature superconducting quantum interference device (High-Tc SQUID) has been developed.
The above susceptibility attenuation immuno-assay method has advantages that the magnetic nanoparticle and the AC magnetic field are never reacted with an antibody to be measured, and the procedures are simplified. However, this method has a disadvantage that the measuring sensitivity totally relies on the detection device itself. In case of the above-mentioned inductive detector using a coil, a resonance circuit using a high Q value is required, but the resonance frequency of the induction coil cannot be easily changed.
The superconducting quantum interference device capable of more precision measurement has a high sensitivity, but since a superconducting phenomenon is employed, it consumes expensive coolant such as liquid helium. Thus, the maintenance cost thereof reaches about 20,000 to 80,000 dollars a year and the like. Accordingly, there is a demand to provide a new detection device not necessitating cooling.